Certain SOC platforms utilize the 8250 serial UART core that do not providevalid modem control lines (in particular, CTS). A mechanism is needed toallow platforms to configure the 8250 driver to disable modem status interrupts.Add a new UPF_NO_MSR flag and use it to check for the condition.

This patch was tested using a MityDSP-L138F platform having a UART CTS pinconnected to a clock signal (and configured, via pinmux, as a clock inputfor another peripheral function).